The present invention generally relates to a method of improving the functions of the surface of alloy steel so as to improve corrosion resistance, adhesion (the properties of bonding metal and organic substances), wear resistance, paintability, weldability, and the colorability of an ornamental color pattern, and the method of this invention is a novel technique which can be widely applied to various industrial fields such as the chemical industry, the machine industry, the automobile industry, and the canning industry in which alloy steel is used as a material. The present invention also relates to an alloy steel and a structure both made by the method. In particular, the present invention pertains to a method in which alloy steel is kept in contact with an aqueous solution of oxidizing acid or salt thereof, the surface of the alloy steel being irradiated with high power laser beam from the outside through the aqueous solution, and the chemical reaction thus caused on the surface being utilized to concentrate a specific component of the alloy steel on the surface thereof, thereby improving the functions of the surface of the alloy steel, and an alloy steel and a structure both made by use of the method of the present invention.
Various proposals have heretofore been made with respect to a method in which the surface of steel is irradiated with a laser beam for the purpose of improving the surface. For example, several methods are known in which the surface of steel is heated to a high temperature by the irradiation of a high-powered CO.sub.2 laser beam directly on the surface of the steel, thereby effecting . the quenching of the steel or alloying of a coating metal provided on the surface of steel, or obtaining an amorphous structure of the surface by rapid heating and quenching. As an example, one of the methods is disclosed in "SURFACE TREATMENT BY USING A LASER SYSTEM," KOGYO ZAIRYO (industrial material) 32 (3) 31 (1984) which is written by Toshihiro Umehara.
The specification of Japanese Patent Unexamined Publication No. 116886/1981 discloses a method in which the surface of a mild steel is irradiated directly with pulse laser beam to activate the surface, thereby improving the characteristics of chemical conversion treatment.
The specification of Japanese Patent Unexamined Publication No. 82780/1980 discloses a method in which a work piece is kept in contact with a gas or a liquid containing halogenide and the surface of the work is corroded by the irradiation of a laser beam thereon, whereby metal combined with halogen (such as W, Fe or the like) is precipitated on the surface of the work.
However, these prior arts never disclose such matter that a particular constituent in the surface of an alloy steel can be concentrated by the irradiation of laser beam onto the surface.
Regarding the coloring of a chromium-containing alloy steel such as stainless steel which coloring is one of the surface functions, the specifications such as of Japanese Patent Unexamined Publication No. 120939/1976 disclose a method of coloring the chromium-containing alloy steel such as stainless steel in which an interference film is formed on the surface of stainless steel by the coloring thereof so that the color of the surface becomes any one of blue, silver, red, purple or green. However, such coloring methods upon which the prior art relies are to monochromatically color the entire surface of a steel sheet, and commonly, it has been difficult to obtain a polychrome pattern. Therefore, the industrial production of polychrome pattern stainless steel has not yet been carried out.